1. Field of the Invention
The present invention relates to transfer devices for transferring an image from an image bearing body such as a photoconductive body or dielectric body to a copy sheet, and, more particularly, to a transfer device for use in an electrophotographic apparatus or electrostatic recording apparatus.
2. Description of the Related Art
In image forming apparatuses such as an electrophotographic apparatus, generally, an electrostatic latent image is formed on a photoconductive body, and a developer image is developed by letting a developer adhere electrostatically to the electrostatic latent image. The developer image may be then transferred to a copy sheet by a transfer device.
There are known transfer means for this purpose, such as, electrostatic means employing a corona transfer method and a roller transfer method, and mechanical means employing an adhesion transfer method.
Since the hazard of ozone generated in the corona discharge process is becoming a public concern, means for removing ozone is generally incorporated into this type of apparatus, or such an apparatus may employ a roller transfer method generating less ozone. However, there is a growing demand for a compact design of this type of apparatus.
The roller transfer method allows a copy sheet to smoothly advance to a transfer position. However, the roller transfer method requires that the copy sheet be pressed against the developer image on a photoconductive body at an appropriate pressure. If the level of pressure is not sufficient, a transfer fault takes place. For this reason, a high machining accuracy and an appropriate softness are required of the photoconductive body. A transfer fault may also take place if the electric resistance of rubber forming the transfer roller is too high. The requirement that makes even more rigorous the selection criterion of the material of the roller is a roller surface property. To withstand repeated uses, the surface of a roller, in direct contact with the photoconductive body and subject to smear, needs to be clean, smooth, and needs to have minimal friction resistance. However, the rubber material used for the rollers typically has a coarse surface and a large friction resistance. Finding a proper rubber material for the surface of the roller which is easy to clean and satisfies elasticity conditions is extremely difficult. For this reason, conventionally, the roller is frequently replaced, rather than employing a cleaning unit, or instead, the apparatus is often provided with a complex cleaning unit. Accordingly, the apparatus has room for improvement in cost and technical viewpoints.
Japanese Patent Laid-Open No. 5-6104 discloses contact-type transfer means as a transfer device. To resolve the problems of pressure and smear, the contact type transfer means employs a slider having a contact made of fiber, rubber or resin, which has both elasticity and electric conductivity, and also employs an AC bias.
In the transfer device disclosed in Japanese Patent Laid-Open 5-6104, setting electrical conductivity and elasticity of the elastic, electrically conductive contact to their proper values is extremely difficult (increasing electrical conductivity in a material tends to increase the hardness of the material). Because the conductive contact makes sliding contact with a photoconductive body, streak scratches occur on the surface of the photoconductive body, due to the hardness of the material. This causes the surface of the photoconductive body to be ground and the life of the photoconductive body to be shortened.